The production of fiber composites using thermally curable impregnated rovings is well known and is taking on increased significance because the fiber composites are light and strong and can be used to form void-free pieces of diverse shape. The production of larger pieces and the use of textile handling machinery are of particular importance. All sorts of problems have been encountered.
The most direct approach is to use dry rovings and to impregnate them with liquid resin on the way to the fiber composite which is being formed. However, aside from the physical difficulty of doing this, if the liquid resin is of low viscosity, then it runs on the fiber composite producing resin-rich and resin-poor zones in the composite. Neither of these is desirable, and both detract from the strength of the products. On the other hand, if the resin is viscous enough to resist running, then it does not adequately penetrate the filaments in the multifilament roving being impregnated, so once again the final product is deficient. Also, thermally curable resins are usually tacky liquids, and the tackiness of the resin causes it to stick to textile machinery so that, for example, the wet-impregnated roving cannot be braided. Removal of impregnant is made more difficult when excess resin is present, and it is difficult to avoid the presence of excess impregnant when the impregnant is viscous and tacky. While impregnation of the roving without storage thereof on the way to the fiber composite is not a prime aspect of this invention, it is improved by this invention.
The use of preimpregnated rovings has also become significant. A prime approach in this direction is the use of a thermoplastic overcoat around the roving which is impregnated with thermosetting resin in semi-solid form. Our coworkers at DeSoto, Inc. have made considerable progress in this direction as illustrated in U.S. Pat. No. 4,220,686 issued Sept. 2, 1980, and its three divisional patents, U.S. Pat. No. 4,187,347 issued Feb. 5, 1980, U.S. Pat. No. 4,195,113 issued Mar. 25, 1980, and U.S. Pat. No. 4,264,655 issued Apr. 28, 1981.
In these disclosures, dry impregnated rovings which usually include a proportion of relatively expensive thermoplastic resin in the thermosetting impregnant, such as a polysulfone resin, are overcoated with a thermoplastic resin coating which is compatible with the impregnated resin system in a hot melt. The thermoplastic resin adds to the expense of the product, organic solvents are normally needed and must be removed, and the impregnation and coating process is slow and adds to the cost of the product. The opportunity to have an appropriately impregnated roving which can be withdrawn from a supply thereof as needed and handled by textile equipment is advantageous, but there are limitations, some of which have been noted.
Wet-impregnated rovings have also been packaged as illustrated in U.S. Pat. No. 4,147,253 issued Apr. 3, 1979. However, the low tack impregnants used in that patent were primarily ultraviolet-curable liquids which are expensive. Also, the ultraviolet-cured products are not as strong as the traditionally used thermally cured products. Also, the unsaturated liquids used in the patent required special way winding to provide voids and roving crossing points to prevent the liquids used from flowing in the package. Such flow is tolerable in the production of fiber composites when ultraviolet light can be used to "freeze" the resin on the formed piece during application of the applied wet-impregnated roving, but this cannot be done easily for thermally curable liquids which do not include polymerizable unsaturation, and this limits the effectiveness of the systems shown in the patent.
The present invention overcomes the various problems noted hereinbefore by using wet-impregnated rovings which are thermally curable and lack polymerizable unsaturation, but which possess low tack at relatively high viscosity even though they can be applied at low viscosity to insure proper penetration of the applied liquid resin in the roving. To do this without employing organic solvent is an important feature of this invention.